The goal of the project is to define a mechanistic basis for a novel therapeutic strategy combining differentiation-inducing agents (e.g., PMA, bryostatin, sodium butyrate, etc.) with pharmacologic cyclin-- dependent kinase inhibitors (e.g., flavopiridol; FP) in leukemia treatment. Recent evidence indicates that co-administration of PMA and FP in leukemic cells dramatically induces apoptosis, markedly reduces clonogenicity, blocks PMA-related maturation, and dysregulates expression of the CDKI p21(WAF1/CIP1). We hypothesize that these actions stem from FP-mediated disruptions in PMA-induced differentiation and/or the associated G, arrest program. Specfically, we propose that FP interferes with PMA-related maturation by 1) blocking the expression of p21(WAF1/CIP1), an inhibitor of apoptosis; 2) accelerating pRB dephosphorylation, thereby dysregulating normal G, arrest events, and 3) disrupting E2F-1/pRB binding, thereby promoting the inappropriate expression of E2F1-related genes that trigger apoptosis. These hypotheses will be tested in a variety of U937 transfectant and parental leukemia cell lines. Evidence of synergism between FP and other differentiation-inducing agents (e.g. sodium butyrate and bryostatin) will also be sought, and features common to these combinations identified. Finally, we will use serial cDNA microarray analysis to identify a spectrum of apoptosis -, differentiation-, and cell cycle-related genes whose expression may be perturbed by PMA/FP co-- treatment. Our ultimate goal is to provide a rational basis for combining differentiation-inducers and CDK inhibitors that may eventually be translated into novel treatment protocols for AML and other malignancies.